There are any number of different designs for the linear or rotary actuators which are used to operate or position various types of mechanical devices such as valves, dampers, machine tools, etc. The force for operation is in most cases supplied either electrically or pneumatically. Most electrical actuators use an electric motor as prime mover driving through some type of speed reduction mechanism to provide the required level of torque or force. A very common design for these actuators has a gear train to reduce the speed and increase the output force or torque. (It is of course well known that linear output can be easily transformed into rotary output and vice versa with mechanical arms or a rack and pinion.)
There is also a class of electrically powered actuators, typically for providing linear output, which uses a hydraulic connection between the electric motor and the output element. In such hydraulic actuators, the electric motor drives a pump which supplies pressurized hydraulic fluid to a cylinder to move a piston sliding therein and to which the actuator's output element is connected. Hydraulic actuators which have been commercially available for several years from the assignee of this application have an electric motor which drives a reciprocating pump having two pumping chambers, allowing two volumes of hydraulic fluid to be ejected to the output cylinder for each rotation of the motor. In this design, the pump is completely submerged.
For certain actuator applications it is important to accurately control the operating speed of the output element. For example, the speed at which a fuel valve for a burner is opened must in some cases be tailored to the particular burner and fuel supply pressure to prevent flame-out and allow smooth lightoff. This means that in many instances, actuators having different operating speeds must be installed in different systems for optimal operation in every case.
For hydraulic actuators, it is axiomatic that speed of output element movement may be changed only by altering the flow rate of the hydraulic fluid supplied to the output element cylinder. For the actuator with the two chamber pump mentioned above, the operating speed can be cut in half by disabling one of the chambers producing the pressurized hydraulic fluid, and this feature has been available for several years. It is also possible to control output element operating speed by altering pump motor speed, but this is relatively expensive and complex. In a variation on this design, the pump motor can be rapidly cycled on and off, varying the motor duty cycle to control operating speed. But motor cycling results in pulsating movement of the output element and can also result in relatively rapid wear rates for the motor and pump. It is also theoretically possible for a supplier to simply provide a number of models of actuators each having a different fixed operating speed. However, this allows only a gross change in the operating speed, and creates nightmarish product stocking and selection problems.
U.S. Pat. No. 5,097,857 (Mayhew) discloses an example of one type of motor driven hydraulic actuator. Mayhew briefly alludes to the desirability of providing variable thrust and/or torque to vary the speed of operation.